In the cable making art there are generally two types of insulation and/or jacket material used in the fabrication of the electrical wire and cable, thermosetting materials and thermoplastic materials. Application of thermosetting materials as the insulation of jacket of an electrical wire or cable requires the use of vulcanization (curing) apparatus to cause the thermosetting reaction to occur. The most widely accepted technique for manufacturing extruded vulcanized type cables is to pass the conductor through a series of extruder heads and apply concentrically the semi-conducting and insulating compounds. After application of the semi-conducting and insulating compounds, the cables are vulcanized under pressure in a saturated steam environment followed by cooling under pressure. In such a steam curing process, the insulated conductor is moved through the vulcanizing apparatus and exposed to pressurized, saturated steam (typically 275 psi) followed by cooling under pressurized water (typically 275 psi). The thermosetting compound contains curing agents which are activated at the high temperatures found within the vulcanization apparatus and the speed of the vulcanization reaction depends on the temperature within the vulcanizing apparatus (for 275 psi steam, approximately 210.degree. C.). Long length vulcanization pipes have been demonstrated to be preferred by those practicing the art of steam curing cables because polyethylene and ethylenepropylene rubber insulations normally have high thermal resistances. As a result, heavily insulated cables of the type used for high voltage operation require long curing time. For this reason a steam curing process is normally used in a horizontal, vertical, slant or catenary continuous vulcanization apparatus. Conventional long steam curing systems are well known in the art.
The conventional CV process is a curing process for insulation being applied to cable wherein the insulation is applied by an extruder and the newly insulated conductor passes into a closed, sealed curing cube. The length of time required for the curing operation is dependent upon time and temperature. As the temperature is elevated in the curing tube, the time required is decreased. Normally, saturated steam is used as a curing media. Saturated steam at 275 psi gives a curing temperature of approximately 410.degree. F. Inside the curing tube, the cable is exposed directly to the saturated steam until it has cured a sufficient time. Before removing the cable from the closed, sealed curing tube, the insulation must be cooled. To accomplish this, cool water is injected in to the lower end of the tube. Thus, approximately 1/3 of the tube is being filled with water, and appromately 2/3 of the tube is being filled with the saturated steam. The purpose of the present steam-water interface control is to control the point where steam and water meet. The location of this point is very critical to the overall curing process because excess water in the tube shortens the time available to cure the cable. Not enough water in the tube results in insufficient cooling time, and does not allow the cable to properly cool before it is pulled through a series of mechanical seals at the end of the tube.
Historically, a heat sensitive thermocouple has been strapped directly to the tube at the estimated area of the liquid-steam interface. This thermocouple in theory senses the change in tube temperature created by the cold water as it is pumped into and up the tube. The thermocouple will generate an electrical signal to shut the pump off and allow the steam to overpressure the water. As the water starts back down the tube away from the thermocouple, the heat of the steam increases the tube temperature and generates a signal that will start the pump again and water is pumped back up the tube. The thermocouple then senses a lowered temperature and will again shut the pump off. Since it is only a heat sensitive device, the thermocouple cannot be relied upon to give a proper water level indication, and therefore the thermocouple is not a reliable indicator of conditions inside the tube. Instead, both water level and temperature should be controlled.
The conventional means of controlling water temperature is a bleed valve placed approximately five to ten feet behind the estimated interface point on the bottom of the tube. This valve is normally a manually controlled valve, and its purpose is to keep the hot steam from overheating the water in the tube. Hot water is bled through the bleed valve to prevent overheating of the cooling section.
Conditions inside this closed tube are similar to the conditions in the radiator of an automobile. Under pressure in a car radiator, water is at temperatures of above atmospheric boiling point (212.degree. F.) but, immediately when the radiator cap is removed steam flashes. A similar condition existed on a much more severe basis inside the curing tube of a CV apparatus since saturated steam inside this tube is at 410.degree. F. and 275 psi. Although the prior art assumed that the water-steam interface occurred at a point in the tube corresponding to an externally detected increase in temperature to above 212.degree. F., the inventors of the present invention realized that water could exist inside this tube at highly elevated temperatures. If this very hot water were present inside the curing tube it would present a problem to the curing process. Even at very high temperatures water does not have the same total BTU content or heat content as would be present with saturated steam. Approximately 2/3 of the energy per pound of steam is the latent energy, the energy required to make the initial water change from a liquid state to a vapor state. Only approximately 1/3 of the total energy in that steam is the heat required to raise this liquid to a specified temperature. Thus, if hot water at temperatures above 212.degree. F. is allowed to exist in the curing tube in quantities that can not be detected by a thermocouple which blindly controls the water level in the tube, the very hot water will impair the curing process because it will displace saturated steam and will prevent the saturated steam from having direct contact with a length of the cable. Although there is some curing effect from the hot water, it is totally insufficient when compared with a steam atmosphere in the corresponding length of the tube.
By checking temperatures at 20 foot intervals down the curing tube, it was determined that the temperature would run 400 degrees, 400 degrees, 400 degrees, and then at a certain point the temperature would rise to about 410.degree. F. and then decrease again to about 400.degree. F. At a point on the tube on supposedly the steam section of the tube a spot always appeared in these readings where the tube temperature was elevated between 10.degree. F. and 20.degree. F. This was only a small section of the tube approximately 10-15 feet of the length of the tube. It was concluded that this temperature spike or increase at a point on the curing tube was a significant indicator of conditions inside the tube. Several CV machines were checked to be sure that this was not a peculiar phenomenom to the one CV line and in every case that was checked, this temperature increase was evident. It was determined that in fact there were two separate interface points.
In the prior art, thermocouples used on the tube were controlling an interface, but it was an interface between hot water and cold water. The second interface point represented by the temperature spike was the liquid-steam interface, was not recognized in the prior art, and was not controlled, but instead was allowed to roam or drift up and down the tube depending upon the open or closed position of the manually operated bleed valve. The temperature spike is the point where the steam-water interface occurs as the latent energy of the steam is being taken off. This represents approximately 2/3 of the total heat content of the steam which results in a temperature increase (net) in the tube temperature at this point where the state change occurs. It was realized that approximately 165 feet of the curing tube was being consumed by hot water which was not detected and could not be controlled by the conventional thermocouple equipment. This 165 feet of hot water represents approximately 100 feet of equivalent curing tube that was being lost which represents approximately about 20% to about 25% of the total cure capacity.